Volvo 819 E - General Principles

Group 28 Ignition systems
Design and function - General
General
cB
o
A
Function of ignition system; combustion theory in brief
Correct ignition tim ing
A. The function of the ignition system is to ignite the compressed fuel/air mixture in each cylinder at exactly the cor-
rect
instant . Ignition is followed by the propagation of a flame fr ont through the combustion chamber until the com-
plete ch arge has been burned. The combustion pressure. whi ch is higher than that of the comp ressed mix ture, gen-
erates a high f orce whi ch drives the pis t on downwards in the cylinder. (Al thou gh a proportion of the energy released
is
converted into mechanical work by the crankshaft, most of the energy in the fuel (approx. 70%) is dissipated in the
form of cooting losses, exhaust gas losses and mechanical losses.)
To ensure that the igni ti on timing is as close as possible to the 'co rrect' setting under all runn i ng conditions, the tim-
ing must be adjusted continuously to take account of variations in the conditions. In other words , the optimum tim-
ing is not a fixed sett i ng for each eng i ne , but varies with factors such as engine speed, load, fuel/ air ratio and temper -
ature . Furthermore , the timi ng may be ad jus ted for optimum comfort, maximum torQue or maximum power as ap -
propriate .
In
the rest of this discussion , the term optimum tim i ng shall be taken to mean a setting which takes account of all the
factors influenced by the ignition. In other words , it is a compromise between the demands of high power , low fuel
consumption and the cleanest possible exhaust emissions. The emergence of ever-stricter emission s tandards has
been
the main factor in the development of igni t ion systems designed to optimize the combus tion process under all
running condi tions.
Earlv ignition
B. Early ignition prevents efficien t expansion of the fuel/ air mixture since the piston is s ti ll travelling upwards to-
wards TOC as the pressure rise occurs, generating an abnormally high pressure accompanied by an excessive tem-
perature rise. As a
consequence of early ignition, part of the unburnt fuel/a ir mixture may i gnite spontaneously,
causing the engine to knock .
In general,
early ignition also has an adverse effect on the exhaust emissions. The Quantity of unburnt hydrocarbons
( HC ) increases, while the pressure and temperature rises cause the nitrogen in the trapped air to react wi th the
oxygen to fOfm greater quantities of nitrogen monoxide (NO) and nitrogen dioxide (NO) Of NO .. oxides of nitrogen) .
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